1. Field of the Invention
This invention relates to a condenser microphone, more particular to a condenser microphone that is formed with a variable gap chamber in a condenser and a closed air chamber between the condenser and a condenser housing and to a method for making the same.
2. Description of the Related Art
FIG. 1 illustrates a conventional condenser microphone 200 disclosed in U.S. Pat. No. 6,243,474. The condenser microphone 200, which is formed by micro-machining technology, includes a microphone membrane unit 2 and a microphone back plate 2′. The microphone membrane unit 2 includes a supporting frame 20, a silicon nitride membrane 21 formed on the supporting frame 20, and a PTFE (polytetrafluoroethylene) electret 23 formed on the silicon nitride membrane 21. The microphone back plate 2′ includes a silicon substrate 24, an insulating layer 22 formed on the silicon substrate 24, and a back plate electrode 26 formed on the insulating layer 22. A plurality of cavities 28 are formed in the silicon substrate 24 and are exposed from the insulating layer 22 and the back plate electrode 26 for reducing air stream resistance during microphone operation.
The conventional condenser microphone is disadvantageous in that formation of the cavities 28 involves patterning and etching processes, and is thus time-consuming and expensive. Since each cavity 28 requires a sufficient depth to effectively reduce the air stream resistance, the thickness of the microphone back plate 2′ is relatively thick. As a consequence, minimization of the size of the condenser microphone 200 is undesirably limited. In addition, attachment of the condenser microphone 200 to a circuit board of an electronic device (not shown) cannot be done by using surface mount techniques (SMT) due to the property of the PTFE electret 23, which is workable at a temperature less than 240° C. Thus, mass production of the aforesaid electronic device is not feasible.
The disclosure of U.S. Pat. No. 6,243,474 is incorporated herein by reference.